This invention relates to a process for the production of 1,4-3,6-dianhydrohexitol isomer mixtures from hexitols. The present invention also relates to isomer mixtures produced by this process and the use of those isomer mixtures as chain-extending agents in the production of polyurethanes.
1,4-3,6-dianhydrohexitols have been known since about 1880. These compounds have been described, for example, in Fauconier, Bull Soc. Chem. (1884) 41, 119; L. F. Wiggins, J. Chem. Soc. 1945, 4; Haworth, Heath, Wiggins, J. Chem. Soc. 1944, 155; R. Montgomery, L. F. Wiggins, J. Chem. Soc. 1947, 433; J. C. Goodwin, J. E. Hodge, D. Weisleder, Carbohydr. Res. 79, 133 (1980); S. Soltzberg, Advances Carbohydr. Chem., 25, 229 (1970); Ropuszynski et al, Przem. Chem. 48, (11), 665-668 (1969); German Offenlegungsschriften No. 3,041,673 (EP-52295) and 3,041,626; and L. F. Wiggins, Adv. Carbohydrate Chem. 5, 191 (1950).
One feature common to all of the known syntheses is that they produce a dianhydrohexitol which corresponds to the hexitol starting material in yields which vary considerably. The main disadvantages of these syntheses include unsatisfactory yields, formation of resin-like reaction residues and, a commercially disadvantageous high crystallinity of the products. Due to the pronounced tendency towards crystallization of the dianhydrohexitols, isomer mixtures which remain liquid at room temperature and do not crystallize cannot be obtained simply by mixing various isomers. Accordingly, if the anhydrohexitols are to be used, in the production of polyurethanes, for example, there is a need to produce them in a solvent-free liquid form because liquids are easier and less complicated to handle during processing. Keeping the substance liquid by fusion, for example, represents an additional energy-consuming process which further complicates processing.
Little is known about the isomerization reactions of dianhydrohexitols from the literature. Only L. W. Wright et al, J. Org. Chem. 29, 2979 (1964) appears to have succeeded in isomerizing dianhydrohexitols by heat treatment at very high temperatures (222.degree.-240.degree. C.) in an autoclave in the presence of hydrogenation catalysts and hydrogen under high pressure (150 atmospheres). However, this disclosed isomerization reaction cannot be successfully carried out in the absence of hydrogen under high pressure.
Applicants' own experiments have shown that dianhydrosorbitol is a suitable unit for the isocyanate polyaddition process, as is dianhydromannitol. However, both substances are solids at room temperature and cannot be prevented from crystallizing, even in mixtures.